Various eye diseases may be treated by various laser treatments. In some of these treatments, such as with laser photocoagulation, there is a visible end point for the treatment e.g. retinal blanching. Subthreshold laser treatment such as selective laser trabeculoplasty described in U.S. Pat. No. 5,549,596 discloses a short pulse laser treatment delivered to the trabecular meshwork providing no visible end point for the treatment. The '596 patent teaches laser pulse durations shorter than the thermal relaxation time of a target tissue in order to confine thermal damage in the target tissue only and to avoid collateral thermal damage. The thermal relaxation time of a particle is related to the particle size. In the case of melanin granules within pigmented trabecular meshwork cells selective cell killing may be achieved with a 532 nm laser at pulse durations of 10 nanoseconds.
Roider teaches (“Microphotocuagulation: Selective Effects of Repetitive Short Laser Pulses”; Vol. 90 pp. 8643-8647, September 1993; Medical Sciences) targeting single RPE cell layer while sparing neural retina by using microseconds laser pulses which are again shorter than the thermal relaxation time of the target tissue. Roider teaches chopping a continuous Argon laser producing 514 nm into microseconds pulses using an acousto-optical modulator. Moreover, in order to avoid negative effects associated with strong local temperature gradient such as cavitation or hemorrhage, Roider teaches inducing additive tissue damage by repetitive short subthreshold pulses, each too small in energy to cause tissue damage by itself. Before a next laser pulse is delivered, heat dissipates to surrounding tissue and the target tissue cools. The heat dissipation out of the target tissue after each laser pulse leads to only a nonsignificant average temperature increase inside adjacent tissue.
Lanzzeta teaches the clinical effectiveness of Non Ophthalmoscopically Visible Photocoagulation (NOVEP). (“Theoretical Bases of Non-Ophthalmoscopically Visible Endpoint Photocoagulation”; Seminars in Ophthalmology; 2001; Vol. 16, No. 1, pp. 8-11). According to Lanzzeta, the target is to raise the temperature of the RPE just to, and without exceeding, the protein-denaturation-threshold. A resulting thermal wave to adjacent tissue will be spread and will reach the neural retina however at a temperature bellow the protein-denaturation-threshold causing no damage however leaving no clinically visible endpoint. Lanzzeta further teaches that a repetitive series of pulses may replace a single pulse using the N-1/4 law for suprathreshold treatment or to be decreased by a factor of 4-10 for a subthreshold treatment. Moreover, Lanzzeta teaches minimizing thermal additivity by not only controlling the pulse “ON” duration, the pulse energy and the peak power, but also by controlling the “OFF” times and duty cycle per second (Hz) so that the temperature rise of the target tissue caused by each pulse is allowed to return to baseline before the arrival of the next pulse.
In order to control the regime of pulses as described above, the '596 patent teaches “a control unit configured to control the irradiating of the tissue with the one or more radiation pulses such that the total radiation energy applied to the tissue provides a sub lethal fluence to the pigmented target cells, thereby selectively photostimulating pigmented cells in the tissue”.
U.S. Pat. No. 7,115,120 also teaches “control over the laser dosimetry to ensure that laser energy reaches the threshold required for RPE cell killing (a therapeutic endpoint), but avoids the administration of laser energies sufficient to damage adjacent cells, such as photoreceptors (collateral damage control)”.
U.S. Pat. No. 5,805,622 discloses an expensive control system for a medical laser which is configured to produce microsecond pulses. The control system requires a very fast feedback loop and several kV which must be applied to a Pockels cell in order to dampen spikes in the laser pulse.
Also known in the prior art for ophthalmological green lasers are fast photodetectors which are configured to sense the output power level of the laser and a software control light loop working on the millisecond scale. A hardware control loop, which is based on the fast photodetector, is designed to provide spike safety protection against high power light spikes.
U.S. Pat. No. 7,771,417 teaches a medical laser which is also configured to deliver microseconds pulses of green laser to provide subthreshold treatment. The '417 patent describes a control system having a two control loops—a first, slow, software light control loop in the millisecond regime and a second, fast, hardware light control loop in the microsecond regime.
It is therefore one aspect of the present invention to provide a simpler control system for a medical green laser which is configured to deliver laser pulses in the microseconds scale.